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Data from: Demographic drivers of age-dependent sexual selection
Martin, Alexandre M.; Festa-Bianchet, Marco; Coltman, David W.; Pelletier, Fanie — 2016-06-02 Sexual selection has a critical role in evolution, and it is fundamental to identify what ecological factors drive its variation. Disentangling the ecological correlates of sexual selection over the long term, however, is challenging and has rarely been done in nature. We sought to assess how demographic changes influenced the intensity, direction and form of sexual selection and whether selective pressures varied with age. We tested whether breeder sex ratio, number of competitors and age structure influenced selection differentials on horn length of wild bighorn rams (Ovis canadensis) of different age classes on Ram Mountain, Alberta. We used 21 years of data including a detailed pedigree, demographic parameters and repeated morphological measurements. Sexual selection on horn length of males of all ages was directional and positive. Selection intensity increased with the number of competitors, reflecting male–male encounter rate during the rut, but was independent of breeder sex ratio or age structure. This result can also be linked to changes in population size because the number of competitors was highly correlated to total number of sheep. This demographic effect likely arises from age-dependent mating tactics. Males aged 2–4 years are weakly competitive and experienced stronger sexual selection as they accounted for a greater proportion of all males. Selection experienced by mature males appeared independent of demography. Our study provides a rare description of the demographic determinants of sexual selection in nature.
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Data from: Comparing measures of breeding inequality and opportunity for selection with sexual selection on a quantitative character in bighorn rams
Martin, Alexandre M.; Festa-Bianchet, Marco; Coltman, David W.; Pelletier, Fanie — 2014-11-25 The reliability and consistency of the many measures proposed to quantify sexual selection have been questioned for decades. Realized selection on quantitative characters measured by the selection differential i was approximated by metrics based on variance in breeding success, using either the opportunity for sexual selection Is or indices of inequality. There is no consensus about which metric best approximates realized selection on sexual characters. Recently, the opportunity for selection on character mean OSM was proposed to quantify the maximum potential selection on characters. Using 21 years of data on bighorn sheep (Ovis canadensis), we investigated the correlations between seven indices of inequality, Is, OSM and i on horn length of males. Bighorn sheep are ideal for this comparison because they are highly polygynous, sexually dimorphic, ram horn length is under strong sexual selection, and we have detailed knowledge of individual breeding success. Different metrics provided conflicting information, potentially leading to spurious conclusions about selection patterns. Iδ, an index of breeding inequality, and to a lesser extent Is, showed the highest correlation with i on horn length, suggesting that these indices document breeding inequality in a selection context. OSM on horn length was strongly correlated with i, Is, and indices of inequality. By integrating information on both realized sexual selection and breeding inequality, OSM appeared to be the best proxy of sexual selection and may be best suited to explore its ecological bases.
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Data from: Sexually antagonistic association between paternal phenotype and offspring viability reinforces total selection on a sexually selected trait
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Martin, Alexandre M.; Festa-Bianchet, Marco; Coltman, David W.; Pelletier, Fanie — 2014-02-17 The evolution of conspicuous sexually selected traits, such as horns or antlers, has fascinated biologists for more than a century. Elaborate traits can only evolve if they substantially increase reproduction, because they probably incur survival costs to the bearer. Total selection on these traits, however, includes sexual selection on sires and viability selection on offspring and can be influenced by changes in each of these components. Non-random associations between paternal phenotype and offspring viability may thus affect total selection on sexually selected traits. Long-term data on wild bighorn sheep (Ovis canadensis) provide the first evidence in nature that association between paternal phenotype and lamb viability strengthens total selection on horn size of adult rams, a sexually selected trait. The association of paternal horn length and offspring viability was sexually antagonistic: long-horned males sired sons with high viability but daughters of low viability. These results shed new light on the evolutionary dynamics of an iconic sexually selected trait and have important implications for sustainable wildlife management.

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(Author: Martin, Alexandre M.)

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